Generally, in the tunnel boring works employing the shield type machine, a steel made cylindrical shield body of the machine is provided at its forward end with a rotary cutter head and at its rearward end with a plurality of shield propelling jacks disposed respectively at peripheral positions of the rearward end. Thus, while tunnel face ground is being bored by means of the rotary cutter head at the forward end of the shield body, the tunneling machine shield is propelled into the ground by means of the jacks, bored tunnel wall appearing right behind the machine shield as a result of the propulsion thereof is retained with a retaining wall comprising a plurality of segments installed as partly overlapping the rearward end of the shield body and a tunnel is thus made in the ground.
In the tunnel boring works with the machine shield of the kind referred to, it has been desirable to control the attitude of the machine shield so that the shield will keep its accurate orientation in alignment with predetermined tunnel boring direction during the propulsion of the shield so as not to cause any deviation of the shield from the predetermined direction.
In order to achieve the above purpose, it is already known to install a laser beam transmitter in bored tunnel behind the machine shield and to equip the shield body with a beam receiving and detecting device which is fixed inside the shield so that, when the shield body deviates from the predetermined boring direction, the position of incidence of the laser beam on the receiving and detecting device also deviates from an appropriate position of the device, the direction and amount of which deviation is detected by the device to produce a positional output signal depending on such detected deviation. The produced signal is provided to a signal processing device to be processed therein so as to indicate through a servo mechanism the deviational direction and amount. Depending on thus indicated deviational direction and amount, the shield propelling jacks are selectively driven so as to correct the attitude of the shield body for returning it to be in alignment with the predetermined boring direction.
In this case, it is desirable that the shield attitude control is performed within a short time since, unless the control is quickly carried into effect, the deviational amount increases and wavy advancing distance of the shield body between control initiation and completion points has to be prolonged. In particular, the deviation of the shield is apt to occur remarkably rapidly when the ground nature changes, so that it is necessary to provide the laser beam receiving and detecting device in an optimum manner for enabling it to scan an expanded detecting range and to have the deviational direction and amount detected quickly and reliably. In addition, at the same time of detecting any deviations of the shield body in horizontal and vertical directions, that is, those in socalled X and Y axes, it is desired to detect deviations with respect to laser beam axis as a pitching angular deviation and directional angular deviation as well as those with respect to the horizon as a rolling angular deviation, whereby attitude conditions of the shield body under the ground will be always grasped for properly controlling the attitude.